1. Field of the Invention
This invention relates to an improved structure for a laminated molded insulation material for insulation of electrical apparatus and machines, and particularly for slot insulation of dynamo-electric machines, and wherein the laminations comprise fiber-reinforced carrier material, utilizing binding media, preferably as interlamination curable (or thermosetting) resins as the interlamination binding material, as well as to a novel method for making such insulation.
2. Description of the Prior Art
It is already known to use for rotor slot insulation of large dynamo-electrical machines primarily laminated glassfiber-reinforced resin ("GFK") molded articles, especially profiles, such as strips, L- and U-profiles, preferably made from hard glass fabrics. Insulations made from such laminated, molded materials are disadvantageous, on the one hand because of their relatively poor inter-laminar adhesion, which becomes apparent by its low resistance to gap formation, and, on the other hand because of their tendency to develop cracks when subjected to compression, bending or tensile stresses, depending on the angle of load, and not only surface cracks but primarily tension cracks such as matrix cracks and cracks along the boundary layer resin/glass, caused by local mechanical overstresses. These deficiencies weaken the electrical properties, especially the dielectric strength of the laminated, molded insulation material. There exists another disadvantage, namely, that the degree of safety preventing an electric breakdown will decrease sharply with a decrease in the number of layers when laminated, molded "GFK" materials are used. This is due to the fact that the probability of a lowering in electric strength of the compound material becomes greater when the number of layers is reduced and surface areas increase because of the many flaws that will occur within the individual layers, such as unfilled spots, pores, conducting inclusions of small-sized impurities and the like.
The stiff, laminated molded insulation is also subjected to unavoidable mechanical stresses during shipment, and also especially during installation, and it is thus not possible to avoid with complete safety formation of cracks caused by local overstresses. Furthermore, the slot insulation, for example, of the rotor component of the machine, is subjected to strong mechanical stresses during the operation of the machine because of the non-uniform heat expansion of components made of copper, insulation and iron and also due to the effects of centrifugal forces etc. In spite of careful electric pre-testing, the installed insulation will often fail the high-voltage test for the reasons mentioned above, requiring re-insulation which in most instances will be very costly.
It has already been proposed, to employ more flexible resin systems possessing greater rupturing elongation characteristics to overcome the disadvantageous susceptility to cracking of the laminated, molded materials. However, this will lower unduly the thermal stability, thus losing the required rigidity of the laminated, molded material. In order to reduce the possibility of an electrical breakdown, profiles of laminated, molded material have been produced by employing a thermally stable insulating foil as an inner layer between plies of glass-fiber fabric. However, this structure for the laminated, molded material is also susceptible to losses in electric properties, similar to profiles made from resins that are 100% glass-fiber-reinforced because the cracks which form in the rigid resin/glass compound are very likely to extend into, and damage the insulating foil.
An attempt was also made to replace the insulating foil by a thicker layer of fleece made from organic or inorganic fibers. It was found that it was possible to reduce the formation of cracks by an appropriate increase in the thickness of the layer but at a cost of mechanical strength and rigidity because of insufficient glass-fiber content of the compounded material. In order to insure the required mechanical strength and rigidity, it becomes thus necessary to increase the thickness of the insulation still further, making the use of the winding slots inefficient and uneconomical. Finally, the attempt had been made to produce the laminated, molded materials from resin/fiber fleeces, including high-compressed material, but this attempt was unsuccessful in most cases because of the great drop in mechanical strength and the loss of rigidity.